Given the variability in response to the novel immunotherapeutic agents and the desire to extend their long- term benefits to more patients, there is an increased need for the development of biomarkers that can predict treatment outcomes, thereby ensure that these expensive new treatments, which may have significant toxicities, are offered to the patients who are more likely to benefit. The FDA approved anti-PD1 agent (pembrolizumab/MK3475) typifies this phenomenon of dramatic therapeutic responses in a subset of patients who cannot be pre-identified, necessitating broad treatment application in an unselected patient population. The objective in this application is to evaluate and validate Bim (a PD-1 downstream signaling molecule) levels in tumor-reactive PD-1+ T cells, as well as soluble B7-H1(PD-L1), as monitoring or predictive biomarkers for response to anti-PD-1 therapy in patients with metastatic melanoma (MM), an excellent model, as PD-1/PD-L1 pathway is an increasingly exploited therapeutic target in this disease and a variety of other cancers, with extremely promising results in clinical trials. Our central hypothesis is that Bim levels in tumor-reactive CD11a high D-1+CD8+ peripheral blood T cells can objectively monitor responses to anti-PD-1 therapy and that excessive release of soluble B7-H1 by the tumor leads to Bim upregulation and treatment resistance in melanoma. This hypothesis will be tested in MM patients undergoing treatment with an anti-PD-1 monoclonal antibody (pembrolizumab) at our Institution. Guided by strong preliminary data, we propose two Specific Aims to test our central hypothesis: 1) Establish the role of Bim for monitoring disease status during anti-PD-1 therapy; and 2) Identify the mechanisms of resistance to anti-PD-1 blockade. In aim 1, an already proven PD-1 downstream signaling molecule Bim (the pro-apoptotic BH3-only protein), which has been established as a feasible marker for the status of PD-1 engagement in the applicants' hands, will be utilized to monitor and predict the T cell responses to anti-PD-1 therapy, as well as identify patients who may achieve late clinical benefit despite radiologic pseudoprogression. Under aim 2, we will explore that soluble B7-H1 (PD-L1) could influence the sensitivity to anti-PD-1 therapy and be a new therapeutic target of dual blocker therapy (anti-PD-1 and anti-PD-L). Our approach is innovative because it utilizes novel markers to assess the reversibility of exhausted anti-tumor PD-1+ T cells and the efficiency of anti-PD-1 blockade, which would directly influence the therapeutic outcome with anti-PD-1 therapy. Our proposed research is significant, because our results could help identify patients with melanoma (and possibly other malignancies) who are most likely to benefit from anti- PD-1 therapy, therefore increasing drug efficacy and decreasing toxicity through a more personalized approach to cancer treatment.